大豆硬脂酰-ACP Δ9脱氢酶 (GmSAD) 基因家族的鉴定及功能分析

Identification and functional analysis of soybean stearoyl-ACP Δ9 desaturase (GmSAD) gene family

摘要:

硬脂酰-ACP Δ9脱氢酶 (Stearoyl-acyl carrier protein Δ9 desaturase，SAD) 在质体中催化单不饱和油酸或棕榈油酸的合成，是控制植物细胞饱和脂肪酸与不饱和脂肪酸比例的关键酶。为解析大豆油酸合成积累调控机制，文中对大豆Glycine max GmSAD家族成员进行全基因组鉴定和保守功能域及理化性质等分析。应用qRT-PCR检测GmSAD各成员的时空表达谱，构建表达载体并通过农杆菌介导烟草Nicotiana tabacum瞬时表达和油酸缺陷型酵母Saccharomyces cerevisiae突变株BY4389遗传转化测试GmSAD酶活性和生物学功能。结果表明，大豆基因组含有5个GmSADs家族成员，其编码酶蛋白均具有二铁中心和SAD酶特有的2个保守组氨酸富集基序(EENRHG和DEKRHE)，预测其活性酶蛋白为同源二聚体。系统进化分析显示5个GmSAD分成2个亚组，分别与拟南芥AtSSI2和AtSAD6亲缘关系较近。GmSAD各成员在大豆根、茎、叶、花和不同发育时期种子等组织中表达谱差异明显，其中GmSAD5在发育种子中、晚期高量表达，与油脂富集时期相吻合。烟草叶片瞬时表达GmSAD5可使叶片组织中油酸和总油脂含量分别提高5.56%和2.73%，而硬脂酸含量相应降低2.46%。缺陷型酵母遗传转化测试显示，过表达GmSAD5能恢复缺陷酵母合成单不饱和油酸的能力和促进油脂积累。总之，大豆GmSAD5对硬脂酸底物选择性较强，能高效催化单不饱和油酸的生物合成，为大豆种子油酸和总油脂积累机制的研究奠定了基础，也可作为油脂品质遗传改良的优异靶标。

Abstract:

Stearoyl-ACP Δ9 desaturase (SAD) catalyzes the synthesis of monounsaturated oleic acid or palmitoleic acid in plastids. SAD is the key enzyme to control the ratio of saturated fatty acids to unsaturated fatty acids in plant cells. In order to analyze the regulation mechanism of soybean oleic acid synthesis, soybean (Glycine max) GmSAD family members were genome-wide identified, and their conserved functional domains and physicochemical properties were also analyzed by bioinformatics tools. The spatiotemporal expression profile of each member of GmSADs was detected by qRT-PCR. The expression vectors of GmSAD5 were constructed. The enzyme activity and biological function of GmSAD5 were examined by Agrobacterium-mediated transient expression in Nicotiana tabacum leaves and genetic transformation of oleic acid-deficient yeast (Saccharomyces cerevisiae) mutant BY4389. Results show that the soybean genome contains five GmSAD family members, all encoding an enzyme protein with diiron center and two conservative histidine enrichment motifs (EENRHG and DEKRHE) specific to SAD enzymes. The active enzyme protein was predicted as a homodimer. Phylogenetic analysis indicated that five GmSADs were divided into two subgroups, which were closely related to AtSSI2 and AtSAD6, respectively. The expression profiles of GmSAD members were significantly different in soybean roots, stems, leaves, flowers, and seeds at different developmental stages. Among them, GmSAD5 expressed highly in the middle and late stages of developmental seeds, which coincided with the oil accumulation period. Transient expression of GmSAD5 in tobacco leaves increased the oleic acid and total oil content in leaf tissue by 5.56% and 2.73%, respectively, while stearic acid content was reduced by 2.46%. Functional complementation assay in defective yeast strain BY4389 demonstrated that overexpression of GmSAD5 was able to restore the synthesis of monounsaturated oleic acid, resulting in high oil accumulation. Taken together, soybean GmSAD5 has strong selectivity to stearic acid substrates and can efficiently catalyze the biosynthesis of monounsaturated oleic acid. It lays the foundation for the study of soybean seed oleic acid and total oil accumulation mechanism, providing an excellent target for genetic improvement of oil quality in soybean.